The exhaust gas emitted from an internal combustion engine, particularly a diesel engine, is a heterogeneous mixture that contains gaseous emissions such as carbon monoxide (“CO”), unburned hydrocarbons (“HC”) and oxides of nitrogen (“NOx”) as well as condensed phase materials (liquids and solids) that constitute particulate matter (“PM”). Catalyst compositions typically disposed on catalyst supports or substrates are provided in an engine exhaust system to convert certain, or all of these exhaust constituents into non-regulated exhaust gas components.
One type of exhaust treatment technology for reducing NOx emissions is a selective catalytic reduction (“SCR”) device. A reductant or diesel exhaust fluid (“DEF”) is typically sprayed or injected into hot exhaust gases upstream of the SCR device. The reductant may be an aqueous urea solution that decomposes to ammonia (“NH3”) in the hot exhaust gases and is absorbed by the SCR device. An SCR monitoring system may be included for determining an efficiency of the SCR device (i.e., a rate at which an SCR catalyst reduces NOx). In order to determine SCR efficiency robustly, the SCR device needs to first be heated to a threshold temperature (e.g., generally between about 240° C. to about 300° C.). However, an exhaust gas system does not always experience elevated temperatures which heat the SCR device to the threshold temperature depending on the specific driving pattern. Typically, the SCR device may not reach the threshold temperature unless the drive cycle is relatively transient (e.g., stop and go driving), the vehicle is relatively heavily loaded with cargo, or during towing. Accordingly, it is desirable to provide a system that elevates the temperature of the exhaust gas such that the SCR device attains the threshold temperature needed to determine SCR efficiency.